Photoisomerization of Protonated Azobenzenes in the Gas Phase.

Because of their high photoisomerization efficiencies, azobenzenes and their functionalized derivatives are used in a broad range of molecular photoswitches. Here, the photochemical properties of the trans isomers of protonated azobenzene (ABH+ ) and protonated 4-aminoazobenzene (NH2 ABH+ ) cations are investigated in the gas phase using a tandem ion mobility spectrometer. Both cations display a strong photoisomerization response across their S1 ← S0 bands, with peaks in their photoisomerization yields at 435 and 525 nm, respectively, red-shifted with respect to the electronic absorption bands of the unprotonated AB and NH2 AB molecules. The experimental results are interpreted with the aid of supporting electronic structure calculations considering the relative stabilities and geometries of the possible isomers and protomers and vertical electronic excitation energies.